Вы бы использовали агрегацию:
select sum(case when code in ('I', 'II', 'III', 'IV', 'V', 'IX') then value else 0 end) as case1,
sum(case when code in ('I', 'II', 'III', 'IX') then value
when code in ('IV', 'V') then -value
else 0
end) as case2
from t;